The invention relates to a process for treating glass substrates, in particular those used as memories for "peripherals" in the field of data processing.
Although the invention is not limited to this application, it will be described with reference to the production of magnetic hard disks.
A magnetic hard disk is generally composed of a support element formed so that it has the shape of a disk pierced at its centre. A series of thin magnetic films, serving for data storage, may in particular be deposited on this disk.
The data is recorded and read using one or more read heads, which are placed above the disk, while the latter is undergoing a rotational movement. In order to achieve high performance in reading the data, the read head must approach the disk as close as possible; one speaks in this case of "contact recording". This is because the signal detected by the head decreases exponentially as its height increases. Moreover, current demands are requiring an ever increasing storage density. This means that the area for storing a given item of information continues to decrease. In order for the recorded data to be read in this way, the distance separating the disk from the read head must continue to decrease; this distance must be less than 300 .ang.ngstroms.
Substrates for producing magnetic hard disks are in particular described in U.S. Pat. No. 5,316,844; these are aluminium substrates. This document also describes an important aspect of these substrates; they must exhibit only very low roughness. This document indicates roughness values, the Rq, or averaged roughness, of which is between 100 and 300 .ang.ngstroms. Current requirements, associated with the growing demand for storage and therefore with an ever decreasing disk/read head distance, correspond to an Rq of less than 20 .ang.ngstroms. The Rq is the averaged roughness measured, in the present case, using an atomic force microscope (AFM) on a 5.times.5 micron square.
Moreover, although the requirements regarding the storage capacity of magnetic hard disks continue to increase, another requirement, which may seem paradoxical, relates to the dimensions of these hard disks.
This is because these data-storage units must take up as little room as possible and also have a low mass. These requirements are associated, on the one hand, with the growing demand for portable, and therefore compact and lightweight, storage units; the development of portable data-processing tools and of software requiring large storage capacities is the basis of this requirement. On the other hand, still for the purpose of increasing data storage capacities, it is advantageous to be able to combine several magnetic hard disks in a given space and therefore to have thinner substrates.
A substrate made of aluminium can have a thickness of less than 0.6 millimeters and at the same time have the required properties for constituting a hard disk, in particular in terms of rigidity and resistance to damage under the impact of the read head against the disk.
In order to remedy these drawbacks and to be able to lighten, and possibly reduce the thickness, of such a substrate, it has been proposed, in particular in Patent Application EP-579,399, to make it from glass. Such a substrate has in particular been produced from a ribbon of glass obtained using the "float" technique, the said ribbon being converted into sheet and finally cut up and shaped into disks to the required dimensions. These are then polished in order to obtain the desired thickness and the desired roughness.
During tests, it has become apparent that these lass substrates are subject to various drawbacks and cannot therefore be used satisfactorily for the production of magnetic hard disks.
In particular, the surface of these glass substrates undergoes a major loss of alkali metals, especially potassium or sodium and essentially the ion provided by chemical toughening. Now, these alkali metals have deleterious effects, especially in the presence of moisture, on the magnetic films deposited on the substrates. This is because it appears that the release of alkali metals into these layers results, in the relatively short term, in the destruction of the data recorded.